Study On Effect Of Asymmetrical Electric Double Layer On Performances Of Thin Film Lubrication

The electric double layer (EDL) about dozens of nanometres thick widely exists in thesolid/liquid interface. In general, there are two solid/lubricant interfaces in a lubricationsystem, i.e. two EDLs accordingly. Some previous studies showed that the influence of theEDLs on the lubrication properties is significant under the condition of lubricating film lessthan100nanometers.The influence of the EDLs on the nano-scale lubrication is usually caused by thefollowing two factors. One is the attractive or repulsive interaction between the two EDLs oflubrication regime which is equivalent to increase or decrease the load. The other is theelectrokinetic effect caused by the EDLs which lead to the phenomena of the electro-viscosityand electro-viscous force. Accordingly, the study of this paper focuses on the influences ofthe interaction between the two EDLs and the electrokinetic effect on the nano-scale thin filmlubrication (TFL).Firstly, a new mathematical model of TFL considering the asymmetrical EDLs effect isdeveloped in the light of the shortcomings of the previous related models, which were usuallybased on the hypothesis that the structures of the two EDLs are symmetrical. Based on therelated researches of the interaction of asymmertrical EDLs, a computational formula of thestreaming potential induced by the asymmetrical EDLs of the two friction surfaces is derivedfrom a new perspective. Then, a modified Reynolds equation with consideration of theasymmetrical EDLs effect containing the corresponding computational formula of theelectro-viscosity is developed. The mathematical model developed in this paper reveals thatthe effect of EDLs actually originates from the "combined action" of the two EDLs regardlessof the respective EDL structures of two friction surfaces. Therefore, the present theory isconcerned with an extension of the EDLs structures from the symmetrical to theasymmetrical.Then, the detailed numerical analyses are carried out by taking the advantage of themathematical model concerning the asymmetrical EDLs effect. The analysis results reveal theregularities of both distributions of the electro-viscosity and the streaming potential which isinduced by the asymmetrical EDLs in the lubrication regime. The results also show that theexistence of the EDLs leads to a noticeable increase in the film thickness of EHL, but theEDLs effect on the EHL pressure is minimal.Besides, there are another achievement that the variations both of the EDL thickness andthe electro-viscosity induced by EDLs with the electrolyte concentration and the temperature were respectively investigated. Then, the EDLs effect on the film thickness and the pressuredistribution of the thermal EHL system consisting of all kinds of ceramic or metallic pointand line contacts with aqueous solution is evaluated by the numerical analysis. It is a greatinnovation in the study of EDLs effect on TFL that considers the thermal phenomenon andheat effect generated in the thermal EHL regime in this paper.Finally, according to the principle of the curve-measurement modes of AFM, theexperimental programs have been planned in order to measure the interaction between twoEDLs by using the force-vs-distance curve mode and the electrokinetic effect caused by EDLsusing the amplitude-vs-distance curve mode of AFM. In addtion, the experimental programsinvolving the symmetrical and the asymmetrical structures of EDLs are established by theprobes and samples which both are the same or the different materials respectively.Subsequently, taking advantage of the apparatus that possess the feature of high precision andhigh resolution, i.e. AFM, the experimental researchs have been carried out to observe thevarious performances of the interaction of EDLs and the electrokinetic effect under thedifferent aqueous solution conditions. The experimental work presented in this paper is aninnovation in the study of the EDLs effects and a significant supplement of the theoreticalresearch of this paper. Furthermore, it provides a new idea for the future related study, and theexperimental results may guide the nano-scale lubricating control under the influence ofEDLs effect.